Two distinct types of power supplies, switching and linear, are used for providing regulated D.C. voltages to, e.g., electrical circuits. Switching power supplies have the advantage of being highly efficient in that they operate at an efficiency which is generally between 70 and 80%. However, switching power supplies have the drawback of being expensive because of the complex circuitry and the high power components which are required. Moreover, heat dissipation is a major problem in switching power supplies because the power devices used are typically allowed to float electrically and, therefore, cannot easily be mounted on heat sinks.
In contrast, the circuitry of a linear power supply is relatively simple and the power devices used are easily mounted on grounded heat sinks since most of the heat to be dissipated is generated by a transformer, rectifier diodes and a series-connected output control transistor. The main disadvantage associated with the use of a linear power supply is that the typical efficiency of about 30% is much lower than the efficiency of a comparable switching power supply.
The efficiency of a linear power supply is dependent upon the difference between the unregulated D.C. supply voltage and the regulated power supply output voltage. As this voltage difference increases, the power dissipation of the power supply control transistor increases and, consequently, the overall efficiency of the power supply decreases.
In U.S. Pat. No. 3,414,802, issued to Harrigan et al, a linear power supply having an improved efficiency is disclosed. This power supply comprises two separate control loops wherein each control loop includes a series control transistor. Each control loop is connected to a different unregulated D.C. voltage source and the outputs of the control loops are connected in parallel across a common load. The nominal output voltage level of the first control loop, which is supplied by the higher of the two unregulated D.C. voltages, is slightly lower than the nominal output voltage level of the second control loop. If the second control loop is operating within its operating range the first control loop will be disabled. If the unregulated D.C. input voltage supplied to the second control loop drops to a level at which it is impossible to maintain the output voltage level, the first control loop will begin to operate. Since the nominal output voltage levels of the two control loops are slightly different there exists a voltage step in the power supply output voltage at the transition point between the two control loop output voltage levels. This voltage step is disadvantageous in applications where an exact output voltage is required. Further, at the transition point the second control loop control transistor is still turned on and is dissipating power. Although the dual control loop power supply disclosed by Harrigan et al achieves a higher efficiency than that of single control loop linear power supplies, this efficiency is still less than that of typical switching power supplies.